I always make sure that I rotate my vegetable crops each year to avoid pests and diseases, that are specific to particular crops building up in the soil. I prepared a seedbed for some parsnips this week that I had used for onions last year. Are there any vegetable crops that do not need to be rotated?

You are doing the best thing by rotaing your crops on an annual basis. As you say, this prevents the build up of pests and diseases in the soil. Rotate as many crops as possible, try not to put two similar crops in the same patch in consecutive years e.g. I would not put Brussels sprouts in a patch occupied by cabbages in the previous year. Obviously there are some crops that you won't be able to rotate because they are permanent fixtures such as Rhubarb, Jerusalem artichokes and Asparagus.